1. Field of the Invention
The present invention relates generally to a filter medium for a turbine including a porous polytetrafluoroethylene (hereinafter, abbreviated as xe2x80x9cPTFExe2x80x9d) membrane. More specifically, this invention relates to a filter medium that is used at an intake side of a gas turbine or a steam turbine installed in a power plant or the like and suited for the collection of airborne particles in the air or a gas.
This application claims priority under the Paris Convention of Japanese patent applications Nos. 2001-179026 and 2001-179027, which are hereby incorporated by reference.
2. Related Background Art
Conventionally, as a filter medium used at an intake side of a turbine, a medium formed by adding a binder to glass fibers and forming a resultant material into a sheet has been in common use. However, the presence of adherent fibrils in such a medium causes self-dusting of the medium when the medium is bent in processing. These fibrils of the glass fibers fall off a filter into a turbine to adhere to a fan. Meanwhile, PTFE is a clean material, and in recent years, a filter medium including a porous PTFE membrane has been brought into use as a high performance filter material to be used for clean rooms in the semiconductor industry. An example of this filter medium is disclosed in JP2000-61280 A.
A filter medium formed of a laminate of the porous PTFE membrane and an air-permeable supporting member exhibits a high collection efficiency compared with a glass fiber medium under the same pressure loss. Therefore, when this filter medium of the laminate type is used, an increase in pressure loss is caused during operation in a shorter time than in the case of using the glass fiber medium. Thus, when used as a filter medium for a turbine, a conventional filter medium using the porous PTFE membrane is increased in pressure loss in a shorter time.
A filter medium for a turbine of a first configuration according to the present invention includes a porous PTFE membrane and an air-permeable supporting member. The air-permeable supporting member has a fiber diameter of not more than 15 xcexcm.
A filter medium for a turbine of a second configuration according to the present invention includes a porous PTFE membrane, a first air-permeable supporting member, and a second air-permeable supporting member. The first air-permeable supporting member and the second air-permeable supporting member are disposed on at least one side of the porous membrane. The first air-permeable supporting member has a fiber diameter smaller than that of the second air-permeable supporting member.
The present invention further provides a method of using the filter medium. In this method, the air-permeable supporting member used in the filter medium of the first configuration or the first air-permeable supporting member and the second air-permeable supporting member used in the filter medium of the second configuration is/are disposed on an upstream side of the porous PTFE membrane with respect to air flow passing through the filter medium toward the turbine.
The present invention further provides a method of producing the filter medium of the second configuration. This method includes steps of: forming an air-permeable supporting member laminate by laminating the first air-permeable supporting member and the second air-permeable supporting member; and laminating this air-permeable supporting member laminate and the porous PTFE membrane or a laminate including the porous PTFE membrane.
In the above description, the fiber diameter of the air-permeable supporting member refers to an average fiber diameter when there are variations in the fiber diameter.